This invention is related to a degassing apparatus for agitating and injecting a gas into molten aluminum to remove hydrogen gas. The apparatus has a shaft which includes a prestressed, tubular shield that is in a state of longitudinal compression as it is being rotated.
Hydrogen gas becomes entrapped in aluminum during the recycling process and must be removed because the aluminum makes a brittle casting. Humidity also reacts with oxygen and becomes aluminum oxide forming end products of aluminum plus hydrogen.
Conventional practice is to agitate the aluminum using nitrogen and/or other gases (argon, chlorine, carbon dioxide) in a process called degassing. Conventional agitating devices have a short life because the heat of the molten aluminum rapidly corrodes or burns many materials used for the shaft connecting the motor to the agitating impeller. Heat resistant graphite tubing is used to protect the shaft, usually with a ceramic shielding. However, high-speed shaft vibrations cause cyclical tensile stresses. Graphite has very poor tensile strength and therefore is unsuitable for high speeds. Graphite also has a very short life (days) because it burns at the metal line.